Ultrasonic liquid atomization has been used so far for spraying a liquid coating product onto a workpiece to be coated. In “Ultrasonic Liquid Atomization Theory and Application” (Partridge Hill Publishers—1998 and 2006) Harvey L. Berger considers the application of electrostatics to ultrasonic liquid atomization and explains that implementing electrostatics onto ultrasonic atomization would theoretically bring a very high efficiency to the control of the spray application, even with voltage much lower than those used in conventional electrostatic spraying.
Since then, some applications of ultrasonic atomization with electrostatics have been considered. For instance, in a paper “Deposition of CuInS2 films by electrostatic field assisted ultrasonic spray pyrolysis” (Solar Energy Materials and Solar Cells 95 (2011) 245-249), Dong-Yeup Li and JunHo Kim discloses a case where a liquid to be sprayed is excited by an ultrasonic probe in order to be converted into an aerosol which is conveyed by a carrying gas towards an outlet of a tube, in order to be used for spray pyrolysis deposition. Such a system does not allow precisely controlling the flow rate nor the density of the aerosol and, in its path between the bath where it is formed and the outlet of the tube, the aerosol might agglomerate, so that the size of the droplets used for coating an object cannot be accurately controlled because of coalescence generally observed between droplets. The coating might be rough or non homogeneous and some defects may be generated on the coated surface.
In “Deposition of Ni-CGO composite anodes by electrostatic assisted ultrasonic spray pyrolysis method” (Materials Research Bulletin 42 (2007) 1674-1682), Jing-Chiang Chen et al. consider creating an aerosol and conveying it in a glass tube toward an nozzle facing a copper plate which supports a substrate to be coated. Here again, there is a risk of agglomeration of the droplets of the aerosol.
These academic works would not be easy to use in an industrial environment because the coating deposition is highly dependent on many factors, such as the geometry and length of a tube conveying an aerosol.
On the other hand, in a known system, the electrostatic field is generally created between an electrode and the object to be coated, this electrostatic field having a field line distribution which is not optimized for the workpiece to be coated. Under such circumstances, a substantial part of the atomized product might end up outside of the surface to be coated, which is detrimental in terms of pollution and from an economic stand point.